1. Technical Field
The present invention relates generally to video projection systems, and more particularly to a gamma correction circuit that adjusts the grey scale distortion in a projected video signal by dynamically changing the projector system gamma transfer function without the need to reset the projector system high and low threshold levels.
2. Discussion
Conventional video display devices exhibit a non-linear light input to light output, or gamma, relationship. Particularly, in liquid crystal light valve (LCLV) projectors, the associated light valves exhibit a non-linear characteristic that is in addition to the normal gamma characteristic of the cathode ray tube used to illuminate the photo sensitive side of the light valve. The resulting output of the video display devices often include color errors caused by imbalances of the above mentioned system gamma characteristics.
Conventional video projectors include controls to adjust video offsets and gains between black and white light levels to correct projector grey scale linearity. Projector grey scale linearity must be maintained, as a projector having an unbalanced or nonlinear grey scale characteristics projects images with fewer shades of grey, thus causing the images to look "washed-out" or "over-saturated" and have poor color qualities. While conventional video projectors are set-up based on low and high level adjustments, errors still exist at the mid-grey levels. An existing gamma correction circuit is disclosed in U.S. Pat. No. 5,461,430 to Hagerman (the "Hagerman patent") which is assigned to the assignee of the present invention and incorporated herein by reference. The gamma correction circuit in the Hagerman patent predistorts the video wave forms to match the general nonlinearity characteristics of the system to correct the nonlinearities associated with the individual light valves and the projector CRT. The circuit in the Hagerman patent primarily corrects the low and high extremes of the gamma function.
While the above gamma correction circuit provides adequate performance characteristics, there is still a need for improvement in the art. Specifically, there is a need for a gamma correction circuit for video display devices that is adjustable to compensate for individual variations in the projector light valves and CRTs. In addition, there is a need for the gamma correction circuit that may be adjusted to affect the nonlinearity compensation in the middle grey scale levels without affecting the low and high levels and therefore minimizes reiteration of projector set-up routines.